I don't have a solution, but I do have a similar problem with the new Belles 150A Reference. The problem did not manifest itself until I attempted a speaker cable upgrade from the shielded LAT SS-800 to the unshielded Xindak FS-2 copper/silver foil. Result was a very loud hum, even with everything but the amp unplugged and the inputs from my 21A pre-amp disconnected. Problem is continuous with the unshielded cable, and is loud enough to make listening to music impossible (all with pre-amp completely removed from the sytem). Problem goes away when my old speaker cables are re-installed (yes, I checked multiple times to make sure I installed the new cables correctly). The hum changes pitch when I move the speaker cables around, but never goes away - I'm no EE genius, but to me, it would seem more logical that this is an EMI problem coming from the amp itself than a ground loop problem, since it goes away with switching only the speaker cables. I also tried lifting the ground from the amp PC with nothing else connected to the amp or plugged in to any outlets on that circuit - hum persisted. I also tried plugging the amp directly into the wall (my normal method) or into my Panamax line conditioner, with no change. I do, however, have dimmer switches installed throughout the house, and I'm not in the mood to replace every switch to see if that might help! Other than this recent challenge, I have been thrilled with the 150A Ref/21A combo. Any other ideas?